UC Davis scientists identify protein key to male fertility

The causes of male infertility can be hard to diagnose, with many tests failing to detect genetic defects. Sometimes, infertility doesn't even involve the genes themselves. It can arise from improper folding of the father's DNA in the sperm. If a couple conceives, this mispackaged DNA can damage the lifelong health of the child.

Paternal health is critical to sperm quality and the health of the offspring. Understanding the packing and folding of DNA in sperm cells is a fundamental question in modern biology."

Satoshi Namekawa, professor of microbiology and molecular genetics, UC Davis

Namekawa and Ph.D. student Yu-Han Yeh have now unveiled an important new piece of this puzzle. They have identified a protein, called DAXX, that guides how sperm DNA is organized. DAXX silences thousands of genes so they don't interfere with reproduction. It also keeps a handful of crucial genes turned on – shaping the delicate, early stages of embryonic development. The work was published recently in Genes & Development. 

This discovery could shed light on the causes of male infertility. It could improve treatments for couples who are struggling to have children. And in the long run, it might even help biologists understand how the father's health and exposure to chemicals can damage the health of future generations.

Packing DNA for the sperm's long swim

The DNA in all human cells is wrapped around thousands of protein spools, called histones. This helps regulate which genes are turned on or off.

Genes can be silenced by twining the DNA tightly around specific histones, or kept "on" by using different histone spools and wrapping the DNA more loosely around them - allowing the cell's machinery to nose its way in and read the genes. Biologists call this the "epigenetic" code, because it is overlaid on the DNA without changing the gene sequences.

By regulating genes, the epigenetic state helps determine whether a cell develops into nerve, muscle, or some other tissue. It is also crucial for reproduction.

"When a baby is conceived, the sperm and eggs transmit not only the genes of the parents, but their epigenetic state, too," Namekawa said. Having the DNA correctly packaged is critical to growing healthy offspring.

The epigenetic pattern is already laid down in an immature round sperm cell. Protein machines comb through the DNA, removing one type of histone spool, H3.4, and replacing them with H3.3 histone spools. Later, as the sperm cell streamlines into its classic tadpole shape, 90 percent of those H3.3 histones are replaced with a smaller spool protein, so the DNA can be further compacted.

Compacting most of the DNA is important because it silences thousands of genes that might interfere with the sperm fertilizing an egg cell. This happens in all of the DNA-containing chromosomes, but especially in the sex-determining X or Y chromosome.

By the time the sperm is ready to swim, its DNA has only a few H3.3 histones left – located at strategic points.

"They may bookmark the genes that will be expressed first in the newly conceived embryo," said Yeh, setting the stage for the body and organs to develop.

This process of swapping histones and bookmarking the sperm DNA can easily go awry, with huge consequences. "We are trying to define the mechanism of how these changes occur," Namekawa said. 

From mice to fertility clinics

Now, Namekawa and Yeh have shown that the DAXX protein plays a key role in this packing process. When male mice had their DAXX gene deleted, Yeh found that fewer H3.3 histones were inserted into their sperm DNA, the animals made fewer sperm, and those sperm were often misshaped.

When Yeh looked closely at the sperm DNA, she found that the sex chromosomes weren't entirely compacted the way they normally are. Genome-wide, over 1,000 genes were abnormally activated, and nearly 2,000 were abnormally turned off.

"So, DAXX plays a double role," Yeh said. "It silences many genes, including on the sex chromosomes, while bookmarking others to remain on."

Yeh and Namekawa found that without DAXX, the abnormal gene expression continued after fertilization, in the growing embryo. This suggested that early development – critical for laying down the body and organ pattern – might be disrupted in the offspring. Consistent with that idea, DAXX-deficient males fathered fewer surviving mouse pups.

Understanding how DAXX shapes epigenetic inheritance could improve how male infertility is diagnosed and treated. Men with low fertility sometimes have abnormal histone patterns in their sperm DNA. 

The new discovery could also help scientists optimize in vitro fertilization methods that sometimes rely on immature sperm cells, whose DNA isn't fully bookmarked. 

"Once you identify this as a problem in human fertility, you can look for ways to fix it," Namekawa said.

Health and obesity across generations

Namekawa and Yeh specifically study sperm development, but their work could potentially help other scientists in the bourgeoning field of "intergenerational health," the study of obesity and other problems being passed from parent to child.

Exposure to "endocrine-disrupting" chemicals such as the insecticide DDT and the antifungal agent vinclozolin may play a role, as animals exposed to these chemicals make fewer sperm, with abnormal histones and gene regulation. 

Their offspring then inherit these abnormal epigenetic states, and later in life develop obesity, kidney disease, and infertility – conditions that might even be passed to subsequent generations. Understanding the role of DAXX could give biologists a new focal point for deciphering how the father's health impacts epigenetic inheritance. 

This research by Namekawa's team is funded by the National Institutes of Health, the Yen Chuang Taiwan Fellowship, and the University of California Davis startup fund.

Additional authors on the paper include: Mengwen Hu (UC Davis and Affiliated Guangdong Second Provincial Hospital, Jinan University, China); Brooke M. Alger, Shruti S. Nene, and Han Wang (UC Davis); and Kai Otsuka and So Maezawa (Tokyo University of Science).